1. Field of the Invention
The present invention relates to a discrete cosine transform (DCT) method, and more particularly to a weighted discrete cosine transform method for use with an apparatus which transform-codes input data, e.g., electronic equipments, such as a digital VTR (video tape recorder) and a variety of data transmission apparatus, for example.
2. Description of the Related Art
Recently, a digital VTR and a transmission apparatus, such as a teleconference system, use a technology for transform-coding image data. One of the most popular transform-coding methods is a discrete cosine transform (DCT) method, for example. As is well known, according to the discrete cosine transform method, input data is transformed into coefficient data ranging from a DC component to a high-order AC component. It is customary that coefficient data thus transformed by the discrete cosine transform method is processed later by some suitable data processing methods, such a weighting and a quantization. The quantization obtains quantized coefficient data by quantizing at a predetermined quantization level the coefficient data obtained when input image data is transform-coded by a transform-coding method, such as the discrete cosine transform or the like.
Coefficient data thus transform-coded and quantized is compressed by a variable length coding using a variable length code, such as a run length code and a Huffman coding. Then, the compressed data is recorded on a recording medium or transmitted to a transmission apparatus.
Upon playback or after the transmission, original image data is obtained by the reverse procedure. Specifically, after the transform-coded data was decoded, the coefficient data obtained in the discrete cosine transform is obtained by inverse-quantizing the decoded data. Then, the original image data is obtained by processing the coefficient data in an inverse discrete cosine transform (IDCT) fashion. That is, a series of the above-mentioned processing is executed in order to reduce an information amount of recorded or transmitted data.
FIG. 1 of the accompanying drawings shows an example of a cosine transform apparatus according to the related art. The cosine transform apparatus will be described below with reference to FIG. 1.
As shown in FIG. 1, data, such as image data to be recorded or transmitted is supplied to an input terminal 1 from a digital VTR or a transmission apparatus body circuit (not shown). The image data supplied to the input terminal 1 is supplied to a cosine transforming circuit 2.
The image data supplied to the cosine transforming circuit 2 through the input terminal 1 is transformed into coefficient data ranging from a DC component to a high-order AC component. Coefficient data thus transformed by the cosine transforming circuit 2 is supplied to a weighting circuit 3. The coefficient data supplied to the weighting circuit 3 from the cosine transforming circuit 2 is multiplied with a predetermined multiplier by the weighting circuit 3 and is thereby weighted. Data weighted by the weighting circuit 3 is supplied to a quantizer 4, in which it is quantized at a predetermined quantization level. Then, quantized data from the quantizer 4 is supplied through an output terminal 5 to the digital VTR or other circuit of the transmission apparatus, e.g. a variable length coder (VLC) using a variable length code, such as a run length code and a Huffman code, though not shown.
The cosine transforming circuit 2 cosine-transforms the input data by the calculation shown by the following equation (1): EQU C=F.multidot.D.multidot.F.sup.T ( 1)
where D is the two-dimensional input data, C is the two-dimensional cosine coefficient and F is the one-dimensional forward cosine transform matrix.
The parameters D, C and F shown in the equation (1) can be expressed by the following equations (2), (3) and (4): ##EQU1##
cfx represent coefficients expressed by the following equation (5). The equation (5) represents coefficient of cosine conversion. ##EQU2## where cf-x=-cfx.
Expressing the inverse transform by the following equation (6), we have the following equation (7): ##EQU3## Specifically, study of the equation (6) reveals that the inverse transform matrix can be obtained by transposing the one-dimensional forward transform matrix F. Incidentally, cf-x=-cfx is established.
As shown by the following equation (8), the weighting circuit 3 weights the cosine coefficients C shown in the foregoing equation (3) by multiplying the weighting coefficient W and the cosine coefficients C shown in the equation(3): ##EQU4## where the weighted result shown in the equation (8) is represented by a weighting value Wyx.
When the coefficient data is obtained by cosine-transforming the input data by the cosine transforming circuit 2 according to the equation (1), the coefficient data is supplied to the weighting circuit 3 and the weighting circuit 3 multiplies the coefficient data with the weighting coefficients shown in the equation (8), the circuit scale of the apparatus is increased and the number of the processing steps is increased.